The present invention relates to controlled source electromagnetic (CSEM) surveying for oil and other hydrocarbon reserves.
CSEM surveying is a valuable research tool used for locating oil and hydrocarbon reserves. In a typical CSEM survey, an electromagnetic (EM) source and a number of EM receivers are located at or near the seafloor. Signals broadcast by the EM source propagate outwards into the overlying seawater and downwards into the seafloor and subterranean strata. Given the typical resistivity of the respective media, propagation occurs by diffusion of electromagnetic fields. Components of the broadcast signal which are detected by the receivers carry information about the electrical properties of the media with which they have interacted. Subsequent analysis of the received signals based on geophysical inversion techniques allows features of the subterranean strata to be identified. For example, regions of high resistivity which are typically associated with hydrocarbon reserves can be identified. To thoroughly survey an area of seafloor, an EM source would typically be towed over an area of interest so that data for a range of source-receiver geometries can be collected.
Although CSEM surveys can provide a valuable insight into the electrical properties of subterranean strata, conventional CSEM surveying and data analysis techniques are subject to a number of limitations.
Current analysis techniques generally require data to be collected using combinations of specialized source and receiver geometries [1, 2, 3, 4]. This is true both in terms of the relative placement of sources and receivers and their relative orientations. The requirement for data to be collected for a number of particular source-receiver geometries imposes restrictions on how much useful data can be collected during towing of an EM source over an array of receivers. Furthermore, since CSEM surveys are often performed by towing a single EM source over an array of receivers multiple times to provide data for different source-receiver orientations, the surveying process for providing data which are amenable to conventional geophysical inversion analysis techniques can be time consuming.
Attempts to address the difficulties associated with providing data more amenable to conventional inversion analysis have been made. For example, specialized EM source configurations have been proposed [5].
A further limitation of conventional CSEM surveying and data analysis techniques is that they do not quickly provide an easily interpretable output representing the electrical properties of subterranean strata beneath a survey area. This means it can be difficult to react to survey results in real time, for example, to focus on a region which may potentially be of particular interest within a larger area. Typical initial results from a CSEM survey comprise curves representing combinations of data from different source-receiver orientations and for different distances between source and receiver pairings [1]. The functional form of these curves is determined by subterranean resistivity variations. However, their form is not determined in a way which is immediately interpretable to provide estimates of the depth, extent, resistivity etc. of possible subterranean hydrocarbon reservoirs. Instead, geophysical inversion techniques are applied to the data represented by the curves to provide, for example, resistivity maps of the subterranean strata. Although this process can result in accurate subterranean models, the timing and computations necessary for fullwave three-dimensional (3-D) inversion of survey data can be very large. It is not unreasonable for a geophysical inversion analysis of survey data to take several days to complete. In addition, the inversion process often requires much a priori knowledge of the subterranean strata configuration and can demand many trial-and-error parameter adjustments. Furthermore, the nature of geophysical inversion techniques means that they generally result in only smooth earth parameterizations.
Accordingly, there is a need for techniques which allow for less geometrically constrained collection of CSEM survey data and for such data to be quickly represented in an easily interpretable way such that the location and boundaries of hydrocarbon reservoirs can be quickly and intuitively identified.